What do you think of adding gnome-disk-utility to the Xfce spin? It's not that big and maybe something could be removed to make room for it?
Lately I've been installing it everytime I boot the live-cd as it's pretty easy to activate my encrypted swap with it (if it was by the command-line, I think I would have to use some cryptsetup command plus some other unknown command to do that ;-) ). I guess it's very useful for other live-cd uses too.
On a related note, usermode-gtk provides usermount ('Disk Management' in 'System' menu) which AFAIK is useless in default Xfce. Its destop file has 'NotShowIn=GNOME;KDE;' May I open a bug report for it to add 'NotShowIn=GNOME;KDE;XFCE;LXDE;' ? (LXDE also doesn't need it because of pcmanfm, right?)